Vented / unvented roof assembly
We are designing a relatively high-performing building envelope and coming up against challenges with the roof assembly. The roof will be a cathedral ceiling (finished interior surface). The challenge is due in part to the steepness (15:12) of the roof. Typically, we would do some combination of exterior rigid and cavity batt insulation, but the steepest tested screw pattern that we can find for the exterior rigid is 12:12. So, I am wondering if we can make a pseudo-vented roof assembly, as per the attached detail. All components of the assembly are vapor permeable. For air-sealing purposes, we would wrap the entire structure with a continuous layer of vapor permeable air barrier (Henry Blueskin). On the roof, we would then create a ventilation space – outboard of the air barrier. Then over battens there would be a continuous plywood layer and a waterproof ice-and-water membrane beneath a standing seam metal roof.
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